Sieve apparatus of this kind, also referred to as vibrating screen sifters, are well known in the art for separating and classifying material into at least two size-fractions by allowing the smaller fraction of the feed material to sift through the screen mesh.
Some apparatus make use of multiple vibrating screens, with a top screen separating the largest or over-size material from the rest of the material, which in turn falls onto an intermediate screen. The intermediate screen mesh then separates an intermediate size of material from undersize material. Other apparatus may consist of two or more housings connected to each other sequentially for retrieving material of different size configurations.
In for instance the food and pharma sectors, hygiene is an increasingly important area of focus when sifting powder or particulate material in a sieve apparatus of this kind. Hence, efficient cleaning is required. Cleaning In Place (CIP) is one approach to cleaning, and for several reasons it is becoming increasingly popular, as the CIP procedure allows for cleaning without handling of heavy screen meshes, possibly with sharp edges, which are furthermore easily damaged.
In order for the sieve apparatus to be able to be cleaned by CIP methods, demands are made to the configuration of the support structure of the screen mesh. In one commonly known solution the screen mesh is supported by a rectangular fixed bar mounted on the side wall. The bar provides a sealing functionality preventing unscreened material to find its way around the screen mesh and into the material of smaller size. However, in addition to difficulties in reaching all areas during the cleaning procedure, the bar can carry finished product all the way to the exit without letting it through the screen mesh. This constitutes a waste and production loss. In a second known solution an elastomer element with packing lip, for instance made of silicone and mounted along the edge of the screen mesh in which case the side wall of the housing is even and without support bars. In the opening between the screen mesh and the silicone edge element, there are obviously potentially severe hygiene challenges. Furthermore, the elastomer edges cannot withstand the chemicals used in CIP cleaning, and break-down after only a few CIP procedures is not uncommon. In order to avoid the critical hygiene problems related to the two above-mentioned, known technologies, a third known solution is today usually used for CIP sieve apparatus. Here, the supporting bars and the seal element are simply removed. Even though this results in a CIP cleanable construction, this solution entails other disadvantages, the main problem being that due to production tolerances of screen mesh as well as the side walls a small gap between the screen mesh and the side wall cannot be eliminated, which in turn allows unsorted material to find its way around the mesh, resulting in residual material in the finished product.